The invention relates to a primer containing a solvent-containing two-component polyurethane binder, which can be applied to a substrate to which an inorganic or organic coating or an inorganic/organic hybrid coating can be subsequently applied.
Polymeric substrates such as plastics are extremely versatile materials with a number of desirable properties. However, a disadvantage of these materials is, for example, their sensitivity to mechanical damage at the surface or to chemicals such as solvents.
One method of protecting the surface of plastics from such damage comprises the application of a suitable coating to the plastics substrate. The composition of the coating primarily depends on whether the surface is intended to be protected from mechanical damage, radiation, the effects of chemicals or other environmental factors (soiling, etc.). Transparent plastics, such as polycarbonate, are particularly sensitive to mechanical-damage on the surface. Therefore, numerous coating materials have been disclosed which effectively protect in particular polycarbonates from mechanical damage. These are substantially organic modified inorganic coatings which usually cure by condensation or UV. Examples can be found in J. Sol-Gel Sci. Techn. 1998, 11, 153-159, Abstr. 23rd Annual Conference in Organic Coatings, 1997, 271-279, EP-A 0 263 428, DE-A 29 14 427 and DE-A 43 38 361.
The application of these inorganic coatings, however, is often associated with the problem that the adhesion between the plastics material and the coating is inadequate. A number of methods for obtaining adequate adhesion have been described in the prior art. Physical methods include plasma or corona treatment and a suitable chemical method could be the use of a primer.
Many primers react with both the plastics surface and with the coating and (covalent) chemical bonds are formed. In the case of polycarbonates as substrate aminosilanes such as aminopropyltri-alkoxysilanes (like in DE-A 19 858 998) are used. Here, the amino group reacts with the polycarbonate surface and the alkoxysilyl group reacts with the organic modified silicon-containing inorganic coating. However, these Nxe2x80x94H-functional primers have the disadvantage that the polycarbonate can be badly damaged by the basic nitrogen function and this damage becomes noticeable, optically, due to an obvious yellow discoloration. Another disadvantage is that the adhesion of inorganic/organic hybrid coatings is rapidly reduced during aging in the presence of water, in particular warm water. For example, the film becomes cloudy, blisters develop and finally the film can separate from the substrate altogether.
It is an object of the present invention to provide a primer for silicon-containing coatings on polymer substrates which enables good adhesion between the organic modified silicon-containing inorganic coating and the surface of the polymer substrate and which does not lead either to optical damage or to instability in the presence of water.
It has now been found that solvent-containing two-component polyurethane binders which contain a curing component containing of an addition product of a polyisocyanate and an alkoxysilane and a lacquer resin which can react with isocyanate groups can be used as primers. These solvent-containing two-component systems represent an ideal combination of very high adhesion between for example a polymer substrate and an inorganic coating and very good weather resistance.
The invention relates to a primer containing a solvent-containing two-component polyurethane binder containing
I. a curing component (A), containing an addition product of at least one organic polyisocyanate (B) with an average NCO functionality of 2.5 to 5.0 and an isocyanate content of 8 to 27 wt. % and an alkoxysilane (C) of formula (I) with at least one isocyanate-reactive group,
Qxe2x80x94Zxe2x80x94SiXaY3xe2x88x92axe2x80x83xe2x80x83(I),
xe2x80x83in which
Q represents an isocyanate-reactive group,
Z represents a linear or branched C1-C12 alkylene group,
X represents a hydrolyzable group,
Y represents identical or different C1-C4 alkyl groups and
a is an integer from 1 to 3, and
II. a lacquer resin (D) which can react with isocyanate groups as adhesion promoter.